The law of superposition poses a prima facie challenge to young-earth creationism. Here are two definitions:
His [Steno's] principle of superposition of strata states that in a sequence of strata, as originally laid down, any stratum is younger than the one on which it rests and older than the one that rests upon it.
Within a sequence of layers of sedimentary rock, the oldest layer is at the base and that the layers are progressively younger with ascending order in the sequence. This is termed the law of superposition and is one of the great general principles of geology.
All things being equal, the law of superposition is a reasonable, commonsense principle. It is, however, easy to come up with examples in which that principle can literally be turned upside down.
For instance, suppose I dig a grave. After I'm finished, I have a pile of dirt. Because I was digging down, the layers are now in reverse order. What was originally the bottom-most layer is now the top-most layer. Each layer is now older than the one on which it rests and younger than the one that rests upon it. The layers are progressively younger in descending order.
Archeology frequently excavates mounds. But what was the order in which layers were heaped on other layers? Does it represent layers of progressive occupation? Or is this the result of digging into the ground to reuse old materials?